The 'Fondo San Celso' is an extraordinarily heterogeneous corpus of drawings from the sacristy of the Milanese sanctuary of Santa Maria near San Celso which was incorporated into the Castello Sforzesco collections in the early 1920s. This collection contains a sheet, previously attributed to the Valsesian painter Cristoforo Martinolio, known as 'il Rocca', which is instead identified here as an autograph work by Lattanzio Gambara. This rather refined study is of particular importance since it relates to the decorative cycle executed by the Brescian painter in 1566 in the collegiate church of Santo Stefano at Vimercate.